Transistors for bendy screens

C&I Issue 4, 2014

Bendable electronic devices could be made cheaper and more resilient using a new type of transistor that makes electronic circuits more forgiving to slight errors in manufacture. This could improve large-area electronics like roll-up LED screens or flexible computer displays and have far-reaching consequences for future biomedical devices, remote sensing, wearable computing and artificial skin.

The new ‘source-gated transistor’ or SGT, made from polysilicon, was originally invented by scientists at the University of Surrey, UK, and technology company Philips. However, researchers in the UK and Holland have now reported significant performance improvements in thin-film digital circuits by using the SGT (Scientific Reports, doi: 10.1038/srep04295).

Transistors act as switches to turn current on and off. They have a control terminal, called a gate, where electric potential is applied, while current flows between a source and a drain electrical node. Current flow depends on the semiconductor material between these two nodes.

‘In source-gated transistors, the structure is exactly the same, but we introduce another means of controlling the current exactly at the source,’ says Radu Sporea, lead researcher at the University of Surrey, UK. ‘This means the semiconductor does not have to control the current anymore because we are already limiting the current as it enters the device.’

Usually, changes to the length of the semiconductor or the geometrics of circuit have a major impact on performance, but the SGTs overcome this vulnerability.  The distance between source and drain no longer matters so much. ‘This is a way of getting robust behaviour despite changes in patterning in a device,’ says Sporea.

Electronic circuits with SGTs should in principle also be more resilient to uncertain logic levels or glitches in the power supply, or even to electronic interference.

In future, low-cost electronic patterning on textiles or plastics might result in slight errors in circuit geometry, but these transistors could cope better with these mistakes.  Also, they are more resistant to extraneous electrical noise from phones or Wi-Fi.

This robustness would suit large-area electronics like thin display screens made on bendable, inexpensive plastic in a roll-to-roll manufacturing process. ‘The main concern with a lot of large-area electronics is their cost and reliability. The problem is that these can be metres across, so how can you make sure the performance is the same across something of that scale,’ says Sporea.

Chemical engineer Zhenan Bao at Stanford University, US, notes that polysilicon cannot be deposited onto plastic substrates, which would require high temperature deposition and laser annealing. She describes the report as ‘an interesting idea,’ but would ‘like to see demonstration of the concept in flexible organic transistors, especially printed organic transistors.’

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